Anything you intend to fire in PMC needs to be fairly clean - inclusions can expand at different rates than the host crystal and cause fractures. Avoid diamonds with "feathers." These are internal fractures, usually along cleavage planes and might get bigger in the kiln. Absolutely no fracture filled goods, avoid laser drilled stones, and inspect for groups of little cracks at the girdle. SI1 is probably the most included I'd choose (and I would select my inclusions carefully) for anything going into a kiln. VS clarity grades are probably better - inclusions are smaller and less severe.
Andrea Robinson Silversmith, Gemologist, Motorcyclist, and Espresso Enthusiast MmeMagpie.com